Many vehicles are used over a wide range of vehicle speeds, including both forward and reverse movement. Some types of engines, however, are capable of operating efficiently only within a narrow range of speeds. Consequently, transmissions capable of efficiently transmitting power at a variety of speed ratios are frequently employed. When the vehicle is at low speed, the transmission is usually operated at a high speed ratio such that it multiplies the engine torque for improved acceleration. At high vehicle speed, operating the transmission at a low speed ratio permits an engine speed associated with quiet, fuel efficient cruising.
The transmission speed ratio may be controlled by adjusting the pressure of hydraulic fluid supplied to various clutches and/or to a variator. Commonly, the fluid is pressurized by a pump driven by the transmission input which, in turn, is driven by the engine crankshaft. The transmission pump syphons power from the engine that could otherwise be used to propel the vehicle. Therefore, it is desirable to minimize the utilization of the pump. Belt variators, which control ratio by adjusting the radius of sheaves, are known to require relatively high fluid flow rates and fluid pressures. Therefore, minimizing the utilization of the pump to supply the pressurized fluid is particularly beneficial in transmissions that employ such variators.